In general, a light emitting diode is formed by sequentially stacking an n-GaN layer doped with n-type impurities, an InGaN active layer and a p-GaN layer doped with p-type impurities on a sapphire substrate. Since there are a great deal of threading dislocations caused by lattice mismatching due to physical properties of GaN or limitations on growth methods of GaN, such a laminated light emitting diode has limitations on light-emission efficiency. Further, the laminated light emitting diode has disadvantages such as a broad spectrum width and a large output variation in addition to the threading dislocations.
To overcome these disadvantages of the laminated light emitting diode, there have been conducted studies on a light emitting diode having a nanostructure in which a p-n junction is formed of one-dimensional nanorods or nanowires. Nanorods grown through the bottom-up scheme greatly reduce threading dislocations that might be produced in a laminated light emitting diode, resulting in improved light-emission efficiency. A light emitting diode employing an array of such nanorods secures higher light output as compared with a laminated light emitting diode.
Meanwhile, to supply electric power to the array of the nanorods, electrodes should be connected to upper and lower portions of the nanorods. Although the nanorods may be grown on a lower electrode so that the nanorods can be electrically connected to the lower electrodes, upper electrodes are formed on the nanorods through an additional process.
An example of a method of forming upper electrodes on an array of nanorods has been disclosed in U.S. Pat. No. 6,882,051 entitled “Nanowires, nanostructures and devices fabricated therefrom” by Majumdar et al. Further, an example of a method of selectively depositing nanometer-sized metal on specific regions of nanorods has been disclosed in International Publication No. WO2004/109815 entitled “Contacts fabric using heterostructure of metal/semiconductor nanorods and fabrication method thereof” by YI et al.
In U.S. Pat. No. 6,882,051 (see FIG. 30), nanowires are formed and then are embedded in a polymer matrix by flowing a polymer solution. Subsequently, the polymer is cured, the polymer matrix is etched until the nanowires are exposed, and then metal is deposited thereon. Accordingly, a metallic bonding pad, i.e., an upper electrode, is formed. According to this method, the nanowires with weak mechanical strength are embedded in the polymer matrix to provide enhanced mechanical strength.
However, in a case where such an array of nanowires is employed to form a light emitting diode, light absorption of the polymer matrix leads to light loss. Thus, light extraction efficiency that relates to extraction of light out of the array of the nanowires is reduced. Additionally, the processes for forming the polymer matrix and etching the polymer matrix are added, resulting in complicated fabrication processes of a light emitting diode.
Meanwhile, in the International Publication No. WO2004/109815, semiconductor nanorods are grown on a predetermined substrate, and metal is deposited on predetermined regions of the nanorods by means of a method such as sputtering or a thermal or electron beam evaporation method. The nanorods and the metal are contacted together such that an ohmic or schottky property is exhibited.
According to the International Publication No. WO2004/109815, a nanometer-sized metal electrode is formed on a certain portion of each of the nanorods so as to provide an electrode structure applicable to various devices.
However, as the nanometer-sized metal electrode is formed on a certain portion of each of the nanorods, an additional metal layer is required to simultaneously drive an array of nanorods. A process of forming such an additional metal layer makes the fabricating method of a light emitting diode complicated.
An object of the present invention is to provide a light emitting diode employing an array of nanorods, which has improved light extraction efficiency.
Another object of the present invention is to provide a light emitting diode employing an array of nanorods, which can be fabricated through simplified processes.
A further object of the present invention is to provide a method of fabricating a light emitting diode employing an array of nanorods and having improved light extraction efficiency.
To achieve these objects of the present invention, the present invention provides a light emitting diode employing an array of nanorods and a method of fabricating the same. A light emitting diode according to an aspect of the present invention comprises an array of semiconductor nanorods positioned on a substrate. An upper electrode layer is deposited and positioned on the array of nanorods such that an empty space remains between adjacent ones of the nanorods. According to the aspect of the present invention, since the space between adjacent ones of the nanorods is not filled with an insulating substance such as a polymer, the light extraction efficiency of the light emitting diode can be improved. Further, since the upper electrode layer is formed by being deposited directly on the array of the nanorods, a method of fabricating the light emitting diode can be simplified.
Each of the nanorods may comprise a first semiconductor nanorod of a first conductive type and a second semiconductor nanorod of a second conductive type. Further, an active layer may be interposed between the first and second semiconductor nanorods.
As the interval between adjacent ones of the second semiconductor nanorods becomes smaller, the upper electrode layer may be formed using a variety of deposition methods.
Preferably, the second semiconductor nanorods may be spaced apart from one another at an interval of 200 nm or less on the average.
The upper electrode layer may be made of a transparent electrode material. Further, an upper electrode pad may be formed on the upper electrode layer. Since the upper electrode layer is made of the transparent electrode material, light emitted from the nanorods can be radiated to the outside by passing through the upper electrode layer.
Additionally, a lower electrode layer of the first conductive type may be interposed between the substrate and the array. The nanorods are grown on the lower electrode layer.
Meanwhile, the lower electrode layer may have an extension portion extending from a side of the array of the nanorods. A lower electrode pad may be formed on the extension portion. Accordingly, bonding wires can be connected to the lower and upper electrode pads to supply electric power so that the array of the nanorods can be driven.
On the contrary, in a case where the substrate is conductive, the lower electrode pad may be formed on a bottom surface of the substrate. Accordingly, a larger number of nanorods can be disposed in one array so that light output can be increased.
A method of fabricating a light emitting diode according to another aspect of the present invention comprises growing nanorods on a substrate. An upper electrode layer is deposited on the nanorods such that an empty space remains between adjacent ones of the nanorods. Accordingly, it is possible to fabricate a light emitting diode with improved light extraction efficiency.
The upper electrode layer may be formed using a variety of deposition techniques such as physical vapor deposition, chemical vapor deposition, electroplating and electroless plating techniques, with considering stepcoverage characteristics thereof. For example, the physical vapor deposition technique may be sputtering, a thermal evaporation method or an electron beam evaporation method. Sputtering, or the thermal or electron beam evaporation method is advantageous to mass production of light emitting diodes, and reduces fabricating costs thereof.
Prior to the growth of the nanorods, a lower electrode layer may be formed on the substrate. The lower electrode layer may be a semiconductor layer of the first conductive type.
Additionally, the lower electrode layer may be exposed by patterning the upper electrode layer and the nanorods. A lower electrode pad may be formed on the exposed portion of the lower electrode layer. Further, an upper electrode pad may be formed on the upper electrode layer.
The nanorods may be grown using for example, MOCVD, MBE, MOHVPE or the like. The nanorods may be grown using such a technique without using a catalytic metal or a template.
According to the present invention, the upper electrode layer is deposited such that an empty space remains between adjacent ones of the nanorods, thereby preventing light absorption by a polymer matrix or the like and thus enhancing the light extraction efficiency of a light emitting diode. Further, since the upper electrode layer can be formed using a simple process, processes of fabricating a light emitting diode can be simplified and fabricating costs can be reduced.